1. Field of the Invention
The present invention relates to a liquid developing apparatus for supplying a liquid developer to a recording sheet on which a latent image is formed, and developing tile latent image by toners of the liquid developer.
2. Description of the Related Art
Conventionally a wet recording apparatus is used to record an image on a recording sheet of a large size such as A0. The wet recording apparatus is so constructed that an electrostatic latent image is formed on the recording sheet serving as an electrostatic recording medium, and developed by bringing a liquid developer containing toners into contact with the recording sheet, thus forming an image on the recording sheet.
FIG. 1 shows a wet recording apparatus capable of recording a colored image, and this apparatus is publicly known and disclosed in Jpn. Pat. Appln. KOKAI Publication No. 1-185569. The apparatus shown in FIG. 1 includes one electrostatic recording head and a plurality of liquid development heads for respective colors. Recording paper is reciprocated between the electrostatic recording head and the liquid development heads to record a color image.
As shown in FIG. 1, an electrostatic recording paper roll 21 is constituted by applying a dielectric layer for holding an electrostatic latent image, to paper of a base. Electrostatic recording paper 20 is carried by the roll 21 in a direction I of arrow 301 (carrying direction).
A multi-stylus electrostatic recording head 22 is provided at a fixed position facing a carrying path for the electrostatic recording paper 20. The head 22 has a number of probes arranged at predetermined pitches in a direction (perpendicular to the carrying direction) crossing the carrying path of the paper 20. A pressing roller 30 for pressing the paper 20 against the head 22, is provided.
Development heads 23a to 23d constituting a liquid development unit are arranged along the carrying path of the recording paper 20. These heads supply liquid developers of different colors and, for example, the heads 23a, 23b and 23c supply liquid developers of cyan, magenta and yellow, respectively.
As illustrated in FIG. 2, the development head 23a (or each of heads 23b to 23d) includes a plurality of liquid developer supply grooves 24 on the upper surface of an elongate block-shaped substrate 23. One end of each groove 24 is connected to a corresponding one (25a) of liquid developer tanks 25a to 25d, and the other end is connected to a corresponding one (26a) of liquid developer pumps 26a to 26d. The liquid developer tanks contain liquid developers of different colors such as cyan, magenta and yellow, and are connected to their corresponding development heads 23a to 23d by means of pipes. The liquid developer pumps 26a to 26d are arranged exclusively for their respective colors, and each of them is provided at the pipe connecting its corresponding one of the development heads 23a to 23d and its corresponding one of the development tanks 25a to 25d.
Returning to FIG. 1, the electrostatic recording paper 20 sent out by the electrostatic recording paper roll 21, reciprocates. Whenever the paper 20 reciprocates, a one-colored image of a multicolored image is recorded on a print region of the paper 20, and this recording is repeated for the number of colors, thus forming a colored image. The electrostatic recording head 22 is common to the respective colors. An electrostatic latent image of one color is formed by the head 22, and then developed using a liquid developer of the color.
Consequently, in order to develop the electrostatic latent image formed on the electrostatic recording paper 20, one of the development heads 23a to 23d, corresponding to the color of the latent image, is lifted up to the carrying path of the recording paper 20 and placed into contact with the paper 20. The other development heads corresponding to the other colors stand by under the carrying path. When the development head contacts the carrying path of the paper 20, the development pump corresponding to the head is operated. The recording paper is thus attracted to the development head by negative pressure generated by the development pump.
If the development head 23a rises to a carrying position of the recording paper 20, the liquid developer pump 26a is operated to cause attraction in the liquid developer supply grooves 24 of the head 23a. Then the recording paper 20 is attracted to the upper surface of the development head 23a by the negative pressure caused by the attraction to stop an opening of the head 23a. Therefore, the negative pressure in the head 23a is increased, and a liquid developer is pumped up from the liquid developer tank 25a and supplied to the liquid developer supply grooves 24. The liquid developer is returned from the development head 23a to the tank 25a through the liquid developer pump 26a.
The recording paper 20 attracted to the grooves 24 is carried through the carrying path in contact with the liquid developer in the grooves 24, with the result that an electrostatic latent image is developed.
When the development of the image of a specified color is completed, the development head 23a separates from the recording paper 20 and lowers to a position under the carrying path of the recording paper 20. Part of the liquid developer supplied to the development head 23a, remains on the surface of the paper 20. A squeeze head 27 is provided to remove the remaining liquid developer and dehydrate the paper 20. The squeeze head 27 is connected to a waste-liquid tank 29 through its dedicated vacuum pump 28. When the vacuum pump 28 is operated, the remaining liquid developer is removed from the recording paper 20 by the negative pressure generated by the vacuum pump 28, and collected in the waste-liquid tank 29.
In FIG. 1, reference numeral 31 indicates a liquid developer concentration detecting device, 32 denotes a high-concentration developer tank for replenishing the liquid developer with high-concentration toners to maintain the constant concentration of the developer, 33 shows an electromagnetic valve to be opened in response to a signal output from the device 31, and 34 represents a vacuum switch.
The liquid developing apparatus having the above constitution is of a multi-stylus electrostatic recording type wherein the multi-stylus electrostatic recording head is employed to record a latent image on the electrostatic recording paper 20. Since, in this apparatus, an electrostatic latent image is formed by discharge caused between the recording paper 20 and the probes, a slight gap is required between the surface of the paper and the probes.
The gap is maintained by particles buried in the surface region of the recording paper 20. However, the unevenness of the paper due to the particles, prevents toners from being uniformly attached to the electrostatic latent image formed on the recording paper 20. In particular, the toners are not attached to the convex portion of the paper, with the result that a formed image includes a subtly white portion, and this portion deteriorates the image.
The smoothness of the surface of the recording paper 20 is decreased (i.e., the roughness is increased) since the surface is uneven and, as a result, discharge is generated. Therefore, a large amount of air may flow into the grooves 24 to generate air bubbles therein and, in this case, a latent image is not partially formed on the recording paper 20 because of the air bubbles, thus causing irregularities in the image.